A therapeutically active ingredient is administered to the human or animal body. The active ingredient contains an α-hydroxy acid, an α-amino acid, a peptide or a protein, the component being radiolabeled with 11C and the radioactive atom replacing the C atom of the carboxyl group or of a peptide bond in the a position. The labeled therapeutically active ingredient which is chemically identical to the active ingredient to be used for therapeutic purposes can be advantageously used in tests which allow a dispersion of the active ingredient to be tested in tissue samples or in the human or animal body by positron emission tomography. The complementary information can for example help to accelerate the approval procedure for medicaments.
